Medicinal mushrooms generally have <5% starch. If a product claims to be pure mushroom then it should also conform to this rule.
Many retail mushroom supplement products are in fact not made from mushrooms. U.S. grown mycelium on grain is commonly sold as "mushroom" when it contains no mushrooms at all. On top of that, the grain on which the mycelium is grown on ends up in the final product, leading to a high amount of starch which also lowers the amount of fungal material present (what you want). Recent independent testinghas shown some "mushroom" products to be over 70% starch and 0% beta-glucans (not good).
Another example of added starch are carriers and flow agents like dextrose and maltodextrin. These cheap additives are often added in to help powders "flow" better during the manufacturing process. Typically very little is required but it can easily be overused. Some testing has shown up to 30% starch from flow agents.
This is why starch testing is such an important, yet simple, test which can show you if what you have is pure mushroom or not. Because the last thing you want is your money going towards starch when you wanted mushrooms.